Conventionally, a system for controlling a motor for vehicle running using an inverter has been employed. In electrically powered vehicles such as an electric vehicle, a hybrid vehicle and a fuel-cell vehicle, an inverter is generally used to control the output torque of the motor for vehicle running. Representatively, the output torque of the motor is controlled by PWM control. In this PWM control, switching elements of the inverter are turned on and off based on voltage comparison between a carrier wave signal (carrier signal) and a voltage command, thereby causing the inverter to apply a pulse width modulation voltage to the motor.
Due to the switching operation during this PWM control, noise occurs in the inverter. Regarding this problem, for example, Japanese Patent Laying-Open No. 6-14557 (PTL 1) discloses a technique by which the frequency of a carrier signal is caused to fluctuate such that the power spectrum density of the fluctuations of this frequency from a prescribed frequency is an inverse function of the carrier frequency, and a carrier signal thereof is applied to a modulation circuit, to obtain a drive signal for an inverter. According to the configuration in PTL 1, it becomes possible to alleviate discomfort from the noise produced by the inverter and to reduce noise.